The overview of the process for preparing hydrogen from water in the supercritical state, according to JP, 2979149, B, issued to the present inventors, is described hereinafter.
The reason why a proportion of hydrogen in a gas does not become high even upon pyrolysis of an organic matter at 650° C. in water in the supercritical state described as above is because carbon monoxide, steam, carbon dioxide, and hydrogen, contained in the gas produced due to reaction between the organic matter and the water in the supercritical state, are in chemical equilibrium state. Such equilibrium reaction can be described by a relationship represented by the following chemical formula:CO+H2O=CO2+H2   (1)
In this case, if an excessive amount of a matter (CaO), reacting with carbon dioxide under a condition of the temperature and pressure of a reactor, and acting to lock a resultant product in solid form while exerting by itself no effect on the relationship of the chemical equilibrium, is caused to coexist at a reactive field beforehand, an amount of the carbon dioxide in the gas will decrease due to reaction thereof with the matter.
In order to maintain the above-described relationship of the chemical equilibrium by coping with a decrease in the amount of the carbon dioxide, a reaction inevitably proceeds in the direction for producing CO2 and H2 due to a reaction between CO and H2O, whereupon CO2 thus produced reacts with an absorbent for carbon dioxide, thereby being removed from the gas.
As a result, there is finally established chemical equilibrium in a form wherein a minute amount of carbon monoxide and carbon dioxide, respectively, and a large amount of steam and hydrogen, respectively, exist in the gas. By cooling the gas and solids (mixture of ash contained in the organic matter, non-reacting portions of absorbent for carbon dioxide, and solid matter resulting from absorption of the carbon dioxide), the steam reverts to water, so that the hydrogen can be separated. Accordingly, it becomes possible to prepare a gas containing hydrogen as a main constituent thereof from an organic matter.
Now, assuming an absorbent for carbon dioxide as X, a chemical reaction formula in this case can be expressed as follows:C+2H2O+X=(XCO2)+2H2   (2)
A hydrogen gas produced originates from water, and an overall chemical reaction formula described as above indicates selective preparation of hydrogen from carbon in an organic matter and water, which can be deemed as a thermochemical decomposition reaction of water.
As a heat source for driving the reaction, heat of combustion, generated upon oxidization of carbon contained in the organic matter, can be used, and besides, since CaO added to a reaction system releases reaction heat upon CaO reacting with water to be turned into Ca(OH)2, such reaction heat as well can naturally be utilized, however, if heat supply is insufficient, heat may be added from outside, thereby promoting the reaction according to the formula (2).
In the case of using the matter X as the absorbent for carbon dioxide, temperature, not lower than a temperature thermodynamically determined, is necessary in order to cause a reaction as expressed by the following chemical formula to occur:X+CO2=XCO2   (3)
Accordingly, a temperature of the reaction system according the invention is inevitably not lower than the temperature at which the reaction according to the formula (3) occurs.
The most recommendable matter as the absorbent for carbon dioxide is CaO or Ca(OH)2.
A hydroxide undergoes dehydration under a high temperature condition as follows:Ca(OH)2=CaO+H2O   (4)Accordingly, a reaction field becomes equal to that in the case of an identical species metal oxide being added at the outset.
Main reactions generate reaction heat described as follows:C+H2O=CO+H2 endothermic reaction 31.4 kcal   (5)H2O+CO=CO2+H2 exothermic reaction−9.9 kcal   (6)CaO+CO2=CaCO3 exothermic reaction−42.5 kcal   (7)
Accordingly, an overall reaction is expressed as follows:C+2H2O+CaO=CaCO3+2 H2 exothermic reaction−21.0 kcal   (8)
It has turned out that the overall reaction is at least a net exothermic reaction, and is expected to proceed on its own from a thermodynamic point of view.
There has since been proposed a process of preparing hydrogen by causing coal powders to react with water in the supercritical state to thereby reduce the water in the supercritical state, wherein CaO in an amount at least sufficient to absorb all carbon dioxide as produced is caused to exist in a reaction system, and thermochemical decomposition of water is implemented substantially without an oxidizing agent added thereto, under a condition of a pressure not lower than 220 atm and a temperature not lower than 600° C.
As a result of studies continued further, the inventors have found out the following and has already submitted an application for patent on the basis thereof under Patent Application No. 2000-112558.
More specifically, since the condition of the pressure not lower than 220 atm and the supercritical state at not lower than 600° C. is quite severe from the viewpoint of apparatus manufacturing and operational safety, the inventors have been searching for a process of preparing hydrogen on a slightly easier condition bearing in mind embodying the invention in the form of an apparatus. The inventors have disclosed such a condition as above in a pending patent application (refer to JP, 2001-302206, A).
That is, in JP, 2001-302206, A, there is described the condition under which 0.5 g of pulverized Taiheiyo coal (coal with carbon content 76%, produced by Taiheiyo Coal Mine) is mixed with 3 g of CaO powders, which is in excess of the equivalent weight, a mixture is charged into a reactor to be heated up to 650° C., and a nitrogen gas from a high pressure nitrogen source is introduced into the reactor, maintaining a pressure inside the reactor at 90 atm. Further, with the elapse of 70 minutes after the introduction of the nitrogen gas, 7 cc of water is fed into a high-pressure steam generator by a pump, steam as generated is fed into the reactor with a temperature maintained at 650° C. to replace the nitrogen gas, and to undergo reaction for 20 minutes. There is further described that reactants are cooled after the reaction to be then fed into a cooler, and are further fed into a gas-liquid separator via a pressure adjuster after solid-liquid separation, thereby sending out a gas to an analytical instrument to measure a volume and species of the gas as formed.
The results of measurements with the passage of time are as shown in FIG. 5. Hydrogen makes up most of the gas as generated with a small amount of methane mixed therein. It is also disclosed that a trace of ethane, carbon monoxide, carbon dioxide, hydrogen sulfide, sulfur dioxide, and so forth are detected.
As is evident from FIG. 5, it has been confirmed that temperature rises up to 700° C. in a few minutes after steam is fed in, resulting in preferential occurrence of a chemical reaction CaO+H2O→Ca (OH)2 (exothermic reaction), and further, it has been confirmed from the result after completion of the reaction that a chemical reaction Ca (OH)2+CO2→CaCO3+H2O (exothermic reaction) has also proceeded.
As a result of continuous and intensive studies made by the inventors, the inventors have come to discover the fact that the following chemical reactions preferentially occur:CaO+H2O→Ca (OH)2 (exothermic reaction)   (9)Ca (OH)2+CO2→CaCO3+H2O (exothermic reaction)   (10)provided that a partial pressure of steam is adjusted under a condition of temperature in a range of 600° C. to 900° C. and a pressure at not lower than 30 atm.
The inventors have tracked the state in which CaO in the reactor undergoes a chemical change into Ca (OH)2 by feeding steam into the reactor while maintaining the temperature of the reactor at 700° C. and varying the pressure thereof from 1 atm to 100 atm.
With the pressure at not higher than 10 atm, CaO is hardly turned into Ca (OH)2. It has been confirmed however that with the pressure at 30 atm, there occurs the exothermic reaction represented by the chemical formula (9), resulting in a rise in the temperature of the reactor. Further, it has been confirmed that there occurs more intensely the reaction at the pressure of 50 atm, 70 atm, and 100 atm, respectively.
Further, those tests have brought about results totally unexpected by the inventors.
More specifically, it has turned out that an effective utilization ratio of CaCO3 produced by way of Ca (OH)2 when causing thermal decomposition CaCO3→CaO for reuse differs markedly as compared with that for CaCO3 produced directly from CaO (CaO→CaCO3) by bypassing Ca (OH)2.
Furthermore, it has been confirmed that a hydrogen yield is not lowered so much as anticipated even if a reaction pressure is lowered down to 30 atm.
After having continued studies still further, the inventors have attempted to provide a process of preparing hydrogen with a high production efficiency by examining from various angles the process of preparing hydrogen, including supply of raw materials, preparation of hydrogen, recovery and circulation of a matter for absorbing carbonic acid gas, generation and consumption of energy, and so forth, in order to design the invention in the form of a more specific plant on those conditions described for preparing hydrogen (refer to JP, 2001-302206, A).
The inventors have confirmed based on the results of tests that it is effective to turn raw material into impalpable powders.
That is, pulverized specimens composed of coal and CaO are charged into a reactor, nitrogen gas is fed from a high-pressure nitrogen source into the reactor while keeping the reactor at a temperature in a range of about 600° C. to 800° C., and a pressure is maintained on the order of from 30 to 60 atm. Thereafter, water in a predetermined amount according to a pump flow meter is fed into a high-pressure steam generator, and steam generated is fed into the reactor at the temperature kept in the range of about 600° C. to 800° C., thereby replacing the nitrogen gas. After completion of the reaction, reactants are fed into a cooler, and after solid-liquid separation, are further fed into a gas-liquid separator via a pressure adjuster, thereby sending out a gas to an analytical instrument.
It has turned out that specimens prepared by pressure-forming pulverized coal and CaO into pellets generate more hydrogen in comparison with specimens prepared simply by pulverizing coal and CaO without pelletization, thus indicating that it is effective to turn raw material into impalpable powders.
However, it is apparent that if coal and CaO in the form of impalpable powders can be fed into the reactor, this will be more efficient than a case of pelletizing coal and CaO after pulverization.
It has turned out that when a fluidized bed of a main reactor is supplied with water under a specific condition, and impalpable powders of CaO and coal, a mixture, composed of the impalpable powders of CaO and coal, reacts most efficiently, so that use is normally made of mixed impalpable powders in a range of 0.005 to 0.05 mm in grain size. However, a drawback with this practice has been found out in that if the mixed impalpable powders are too fine in gain size, the same will be carried away on air current out of the main reactor.
As a result of trying various processes as for conditions causing grain growth to occur in the main reactor even if the mixed impalpable powders are small in grain size, it has been found out that grain growth as desired occurs to the impalpable powders of coal and CaO by causing the mixed impalpable powders of coal powders and CaO to undergo grain growth in the fluidized bed while adjusting a steam partial pressure in the main reactor, thereby leading to development of the present invention.